internal/encoding/tag: support marshaling weak fields

We already support unmarshaling weak fields, support the other direction.

Change-Id: I514e6b7b18cf9a3567fb1775a1e389fe64f22d22
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/185340
Reviewed-by: Herbie Ong <herbie@google.com>

internal/encoding/tag/tag.go

@@ -170,7 +170,9 @@ func Marshal(fd pref.FieldDescriptor, enumName string) string {
 	if fd.IsPacked() {
 		tag = append(tag, "packed")
 	}
-	// TODO: Weak fields?
+	if fd.IsWeak() {
+		tag = append(tag, "weak="+string(fd.Message().FullName()))
+	}
 	name := string(fd.Name())
 	if fd.Kind() == pref.GroupKind {
 		// The name of the FieldDescriptor for a group field is

internal/testprotos/test/test.pb.go

@@ -1469,7 +1469,7 @@ func (x *TestRequiredGroupFields) GetRepeatedgroup() []*TestRequiredGroupFields_
 }
 
 type TestWeak struct {
-	WeakMessage   *weak.WeakImportMessage `protobuf:"bytes,1,opt,name=weak_message,json=weakMessage" json:"weak_message,omitempty"`
+	WeakMessage   *weak.WeakImportMessage `protobuf:"bytes,1,opt,weak=goproto.proto.test.weak.WeakImportMessage,name=weak_message,json=weakMessage" json:"weak_message,omitempty"`
 	sizeCache     protoimpl.SizeCache
 	unknownFields protoimpl.UnknownFields
 }